Presented By :-Er. Srishtee ChaudharyLecturer E.C.EGPCG,Patiala RECEIVERS

RECEIVERS Radio receiver is an electronic equipment which pick ups the desired signal, reject the unwanted signal and demodulate the carrier signal to get back the original modulating signal.

Function of Radio ReceiversIntercept the incoming modulated signalSelect desired signal and reject unwanted signalsAmplify selected R.F signalDetect modulated signal to get back original modulating signalAmplify modulating frequency signal

Design of Receiver

The radio receiver has to be cost effectiveRequirements:Has to work according to application as for AM or FM signalsTune to and amplify desired radio stationFilter out all other stationsDemodulator has to work with all radio stations regardless of carrier frequency

Classification of Radio Receivers

Depending upon application AM Receivers - receive broadcast of speech or music from AM transmitters which operate on long wave, medium wave or short wave bands.FM Receivers receive broadcast programs from FM transmitters which operate in VHF or UHF bands.

Communication Receivers - used for reception of telegraph and short wave telephone signals.

Television Receivers - used to receive television broadcast in VHF or UHF bands.

Radar Receivers used to receive radio detection and ranging signals.

Depending upon fundamental aspectsTuned Radio Frequency (TRF)ReceiversSuper-heterodyne Receivers

RECEIVERS Tuned Radio Frequency (TRF) Receiver:Composed of RF amplifiers and detectors. No frequency conversionIt is not often used.Difficult to design tunable RF stages.Difficult to obtain high gain RF amplifiers

Super-hetrodyne ReceiverDownconvert RF signal to lower IF frequencyMain amplifixcation takes place at IF

Communication ReceiverDownconvert RF signal to two IF frequency

TRF (Tuned Radio frequency) RECEIVER

TRF receiver includes an RF stage a detector stage and an audio stage .Two or three RF amplifiers are required to filter and amplify the received signal to a level sufficient to drive the detector stage.

RF section (Receiver front end) used to detect the signalbandlimit the received RF signal and amplifying the received RF signal.AM detectorDemodulates the AM wave and converts it to the original information signal.Audio sectionUsed to amplify the recovered signal

Advantages of TRFTRF receivers are simple to design and allow the broadcast frequency 535 KHz to 1640 KHz. High senstivity.

Disadvantages of TRFAt the higher frequency, it produces difficulty in design.It has poor audio quality.Drawbacks InstabilityVariation in BWPoor Selectivity

INSTABILITYDue to high frequency, multi stage amplifiers are susceptible to breaking into oscillation.As gain of RF amplifier is very high ,a small feedback from output to input with correct phase can lead to oscillations.Correct phase means a positive feedback and it takes place due through stray capacitancesAs reactance of stray capacitances decreases at higher frequencies resulting in increased feedback.Forcing the device to work as an oscillator instead of an amplifier.

VARIATION IN BANDWIDTH

The bandwidth is inconsistent and varies with the center frequency when tuned over a wide range of input frequencies.As frequency increases, the bandwidth ( f/Q) increases. Thus, the selectivity of the input filter changes over any appreciable range of input frequencies.

Example Suppose required BW=10KHzWe have f1=545KHz,f2=1640KHzQ1= f1/BW= 54.5 , Q2=f2/BW=164 But practically Q is limited upto 120Considering Q limit 120 , BW changes to13.6 KHz ( as BW=f2/Q2=1640/120)So Adjacent channel is picked up resulting in variation in bandwidth.

POOR SELECTIVITY The gains are not uniform over a very wide frequency range.Due to higher frequencies ability to select desired signal is affected.

Due to these drawbacks TRF are rarely used.

SUPER HETRODYNE RECEIVERThe shortcomings of the TRF receiver are overcome by the super heterodyne receiver.

Heterodyne to mix two frequencies together in a nonlinear device or to transmit one frequency to another using nonlinear mixing. Also known as frequency conversion , high frequency down converted to low frequency.(IF)A super heterodyne receiver converts all incoming radio frequency (RF) signals to a lower frequency known as an intermediate frequency (IF).

DRAWBACKS OVERCOMEDStability as high frequency is down converted to IF the reactance of stray capacitances will not decrease as it was at higher frequencies resulting in increased feedback.No variation in BW- as IF range is 438 to 465 KHz (in case of AM receivers) mostly 455KHz ,appropriate for Q limit (120).Better selectivity- as no adjacent channels are picked due to variation in BW.

RF sectionConsists of a pre-selector and an amplifierPre-selector is a broad-tuned bandpass filter with an adjustable center frequency used to reject unwanted radio frequency and to reduce the noise bandwidth.RF amplifier determines the sensitivity of the receiver and a predominant factor in determining the noise figure for the receiver.

Mixer/converter sectionConsists of a radio-frequency oscillator and a mixer.Choice of oscillator depends on the stability and accuracy desired.Mixer is a nonlinear device to convert radio frequency to intermediate frequencies (i.e. heterodyning process).The shape of the envelope, the bandwidth and the original information contained in the envelope remains unchanged although the carrier and sideband frequencies are translated from RF to IF.

IF sectionConsists of a series of IF amplifiers and bandpass filters to achieve most of the receiver gain and selectivity.The IF is always lower than the RF because it is easier and less expensive to construct high-gain, stable amplifiers for low frequency signals.IF amplifiers are also less likely to oscillate than their RF counterparts.

Detector sectionTo convert the IF signals back to the original source information (demodulation).Can be as simple as a single diode or as complex as a PLL or balanced demodulator. Audio amplifier sectionComprises several cascaded audio amplifiers and one or more speakers

AGC ( Automatic Gain Control )Adjust the IF amplifier gain according to signal level(to the average amplitude signal almost constant). AGC is a system by means of which the overall gain of radio receiver is varied automatically with the variations in the strength of received signals, to maintain the output constant.

AGC circuit is used to adjust and stabilize the frequency of local oscillator.Types of AGC No AGCSimple AGCDelayed AGC

FREQUENCY CONVERSION in the mixer stage is identical to the frequency conversion in the modulator except that in the receiver, the frequencies are down-converted rather that up-converted.In the mixer, RF signals are combined with the local oscillator frequencyThe local oscillator is designed such that its frequency of oscillation is always above or below the desired RF carrier by an amount equal to the IF center frequency.Therefore the difference of RF and oscillator frequency is always equal to the IF frequency

The adjustment for the center frequency of the pre-selector and the local oscillator frequency are gang-tune (the two adjustments are tied together so that single adjustment will change the center frequency of the pre-selector and at the same time change the local oscillator)when local oscillator frequency is tuned above the RF high side injectionwhen local oscillator frequency is tuned below the RF low side injectionMathematically expressed :High side injectionLow side injection

COMPARISONTRF ReceiverSuper hetrodyne ReceiverNo frequency conversionNo IF frequency Instability , variation in BW and poor selectivity due to high frequenciesDifficult to design tunable RF stages.Rarely usedFrequency conversionDownconvert RF signal to lower IF frequencyNo instability, variation in BW and poor selectivity as IF introduced.Main amplifixcation takes place at IFMostly used

CHARACTERISTICS OF RADIO RECEIVERSSensitivity

Selectivity

Fidelity

SensitivityAbility to amplify weak signals.Minimum RF signal level that can be detected at the input to the receiver and still produce a usable demodulated information signal.Broadcast receivers/ radio receivers should have reasonably high sensitivity so that it may have good response to the desired signal But should not have excessively high sensitivity otherwise it will pick up all undesired noise signals. It is function of receiver gain and measures in decibels.

Sensitivity of a receiver is expressed in microvolts of the received signal.Typical sensitivity for commercial broadcast-band AM receiver is 50 V.Sensitivity of the receiver depends on :Noise power present at the input to the receiverReceiver noise figureBandwidth improvement factor of the receiver

The best way to improve the sensitivity is toreduce the noise level.

Selectivity Selectivity of radio receiver is its ability to differentiate desired signal from unwanted signals.

Selectivity is obtained by using tuned circuits, which are tuned to desired frequency. The quality factor of these LC circuits determines the selectivity. It is given by, Q=XL/R

For better selectivity Q should be high.

FidelityFidelity is defined as a measure of the ability of a communication system to produce an exact replica of the original source information at the output of the receiver.Any variations in the demodulated signal that are not in the original information signal is considered as distortion.Radio receiver should have high fidelity or accuracy.Example- In an A.M. broadcast the maximum audio frequency is 5 KHz hence receiver with good fidelity must produce entire frequency up to 5KHz.

IMAGE FREQUENCYIn radio reception using heterodyning in the tuning process, an undesired input frequency that is capable of producing the same intermediate frequency (IF) that the desired input frequency produces. Image frequency any frequency other than the selected radio frequency carrier that will produce a cross-product frequency that is equal to the intermediate frequency if allowed to enter a receiver and mix with the local oscillator.It is given by signal frequency plus twice the intermediate frequency fsi = fs + 2fi

It is equivalent to a second radio frequency that will produce an IF that will interfere with the IF from the desired radio frequency.if the selected RF carrier and its image frequency enter a receiver at a same time, they both mix with the local oscillator frequency and produce different frequencies that are equal to the IF.Consequently, two different stations are received and demodulated simultaneously

The higher the IF, the farther away the image frequency is from the desired radio frequency. Therefore, for better image frequency rejection, a high IF is preferred. However, the higher the IF, it is more difficult to build a stable amplifier with high gain. i.e. there is a trade-off when selecting the IF for a radio receiver (image frequency rejection vs IF gain and stability)

MATHEMATICAL ANALYSISBasic principle with two frequencies component f1 and f2,we have harmonics f1,f2,f1+f2,f1-f2In case of radio receivers, two frequency components are fo and fsSo harmonic we have fo,fs,fo+fs,fo-fsLet Undesired frequency fsi(=fo+fif) able to reach at the mixerSo now two frequency components will be fo (local oscillator) and fsi (undesired freq)And harmonics will be fo,fsi,fo+fsi,fo-fsi

And harmonics will be fo,fsi,fo+fsi,fo-fsiSubstituting value of fsi we have fo,fo+fs,2fo+fif,fifIt was observed that difference component is a mirror image of IFConsequently, 2 different stations are received and demodulated simultaneously

Once an image frequency has down-converted to IF, it cannot be removed. In order to reject the image frequency, it has to be blocked prior to the mixer stage. i.e. the bandwidth of the pre-selector must be sufficiently narrow to prevent image frequency from entering the receiver.

CHOICE OF IFVery high IF will result in poor selectivity and poor adjacent channel rejection

A high value of IF will result in tracking difficulties

At low values of IF image frequency rejection is poor. Also the selectivity will be too sharp that cut off the sidebands

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